1. Field of the Invention
This invention relates to focusing methods for zoom lenses, and, more particularly, to the use of a lens component nearer to the image plane in focusing, or a so-called rear focusing method.
2. Description of the Prior Art
In the art of zoom lenses of the general type comprising three or four components, or so-called 3-component and 4-component zoom lenses, there have been known many proposals for employing one of the components in the rear assembly, behind the zoom components, for focusing purposes, or the so-called rear focusing method. This rear focusing method is particularly advantageous when employed in the optical systems of autofocus cameras. That is, owing to this method, the focusing lens is small in bulk and light in weight, which is very advantageous at speeding up the focusing operation and reducing the load on the drive system for focusing the lens.
The zoom lenses employing such focusing method is, however, generally accompanied with the following drawback, which will be explained by reference to FIG. 1 where the inconvenient situation of the movable components for zooming and focusing encountered when focusing is followed by zooming is depicted.
In the developed view of FIG. 1, line V--V' represents the path of movement of a component for variation of the image magnification when zoomed in front of a focusing component, line P--P' the path of movement of the focusing component when zooming with an object at the shortest possible distance, and line Q--Q' the path of movement of the focusing component when zooming with an object at infinity. Now assuming that the focusing component lies at a point A with the zoom component at a station Z1 to effect focusing to a finite object distance. When zooming from that position toward the wide angle position, since the shape of the camming slot for the focusing component provided in the zoom control sleeve is conformed to the path of movement of the focusing component for an infinitely distant object, the focusing component moves in a path parallel with the line Q--Q', reaching a point A'. For the given object distance, however, the in-focus position is jumped to a point A". To perform focusing, therefore, the focusing component has to be moved from the point A' to the point A". It is in the camera system having an automatic focusing function that a focusing condition for the point A' is detected to produce a signal by which the operation of the drive motor is then controlled to adjust the focusing component to the point A". In essence, however, for every object in the prescribed range of distances from inifinity to a certain shortest one, what position the focusing component necessarily takes to effect focusing can be found to exist in a quadrangular area PP'Q'Q of FIG. 1, while the point A' representing an object distance in a closeup range beyond the above-defined focusing range. Thus, a large proportion of the focusing movement from the point A' to the point A" is of no significance in principle. This implies that the establishment of the in-focus condition is delayed. Further, there is some possibility of occurrence of a focusing control signal of which the level is either zero or only noises when the point A' is detected, contributing to a cause of faulty operation of the automatic focussing apparatus.
It is known to provide means for preventing this focusing lens from being moved away from that space in which it should exist for the normal focusing range by zooming in Japanese Laid-Open Patent Application No. Sho 56-162727. According to this prior art, use is made of a member having a shape in conformity with the curve PP' shown in FIG. 1 as arranged in the mounting mechanism so that this member plays the role of a stopper, thus achieving the maintenance of the focusing lens in the space that is limited to a longitudinal length and a location depending upon the zooming position despite focusing being followed by zooming to any position. However, the difficulty of when to incorporate such non-linear restricting member into the mechanical mounting for the lens system, the difficulty of finely adjusting the position of the restricting member, and, further, the complexity of structure of the mechanical mounting, are very much increased.